The present invention pertains to improvements in the field of corrosion protection. More particularly, the invention relates to a method of protecting metal surfaces and metal articles against corrosion, as well as to a method of preventing the corrosion of metal reinforcing bars in reinforced concrete structures.
In the corrosion prevention of steel and other metals susceptible to corrosion, the metal is separated from the corrosive environment by a variety of barriers. Such barriers reduce the rate of transport of corrosive substances to the metal surface. These barriers are applied as wrappings or coatings to the metal surface.
For example, metallic coatings are applied to steel by dipping the steel into the molten metal, spraying the molten metal onto the steel surface or electroplating the metal onto the steel. Usually the metallic layer thus formed is of sufficient thickness to be impermeable to oxygen and water vapor. This layer excludes corrosive substances and prevents the corrosion of the underlying steel surface. Examples of metals which may be used for coating steel include zinc, aluminum and chromium. Such conventional metallic coatings have the disadvantage of inadequate adhesion to rusted or contaminated steel surfaces and are therefore precluded for corrosion protection in situations where adequate surface cleaning is not possible.
In the corrosion protection of steel where abrasive blast cleaning is not possible, petrolatum tapes are used for corrosion protection. Fabric is saturated with petrolatum materials, manufactured into flat sheets and rolled into tape form. These petrolatum based tapes are applied to the steel surface, usually being wrapped around steel pipes and tubes. Due to the thickness and impermeability of the petrolatum tape, the transport of corrosive substances including moisture and oxygen to the steel substrate is substantially retarded so that the rate of corrosion is substantially reduced. These petrolatum tapes have the disadvantages of low adhesion to the substrate. The tapes do not adhere sufficiently to support there own weight and must therefore be applied by circumferential wrapping around tubular objects. These tapes are soft and weak, being susceptible to damage by peeling, impact and abrasion. Another disadvantage of these tapes is the excessive thickness required and lumpy uneven appearance and color. These tapes are therefore precluded for corrosion protection in situations where circumferential wrapping is not possible, in situations where a paint-like appearance is desired and in situations where mechanical impact and abrasion occur.
In the corrosion protection of steel, coatings consisting of pigmented organic or inorganic resins are also applied as fluids to steel surfaces. After application, the resins solidify to form continuous semipermeable membranes adhered to the steel substrate. These resins are substantially impermeable to dissolved salts such as chloride ions, and are permeable to oxygen and water vapor. Pigments consisting of flake shaped particles are incorporated into these coatings. These pigments being themselves impermeable to oxygen and water vapor, reduce the vapor permeability of the coatings. Examples of such pigments include glass, aluminum or mica flakes. Although these flake pigments reduce the rate of oxygen and water vapor diffusion through the coatings, they do not render the coatings completely impermeable to vapor diffusion. The pigmented resinous coatings provide effective corrosion protection to steel surfaces which have been thoroughly cleaned of mill scale, rust, soluble salts and other contaminants. It is universally accepted that thorough surface preparation is critical to the long term successful performance of these coatings. The durability or service life of the coatings is proportional to the degree of cleanliness of the steel surface. Increasing the degree of surface cleanliness increases the coating service life. In corrosion theory, it is believed that contaminants under the coating at the metal surface catalyze the electrochemical corrosion reactions.
Organic coatings based on aluminum pigmented epoxy resins and specially formulated for application to rusted and contaminated steel surfaces have been developed. These are known in the trade as "surface tolerant coatings". It is well known that these surface tolerant coatings, while less sensitive to the standard of surface preparation, do not provide satisfactory durability in highly corrosive environments. These coatings fail within an unacceptably short period, usually within two years, if applied to rusted or contaminated steel and exposed to severely corrosive environments. The mode of failure of these coatings is blistering followed by detachment caused by the ongoing process of metal corrosion beneath the coating.
Standards for surface cleanliness of steel have been adopted by organizations involved in the corrosion protection of steel. One such standard is ISO 8501-1 issued by the International Organization for Standards. This standard defines the following degrees of cleanliness for steel listed in order of increasing degree of cleanliness:
St 2: hand or power tool cleaning where tightly adhering rust and mill scale remains on the steel.
St 3: thorough hand or power tool cleaning to bright metal appearance.
Sa 1: light blast cleaning known as Brush-off blasting.
Sa 2: thorough blast cleaning known as Commercial blast cleaning.
Sa 21/2: very thorough blast cleaning. Known as a Near-White metal appearance.
Sa 3: blast cleaning to visually clean steel.
Known as a White metal finish.
A coating applied to a surface cleaned to standard ISO 8501-1 Sa 3 (white metal) will have a significantly longer service life in comparison to the same coating applied to a surface prepared to the lower standard St 2 (adherent rust remains). Surface preparation usually represents more than half the total cost of a steel coating project. Increasing the standard of surface preparation increases the cost of surface preparation. Prior art coatings have the disadvantage of being sensitive to the standard of surface preparation.
In certain circumstances, particularly in maintenance of steel structures, surface preparation by abrasive blast cleaning is impossible or not cost effective due to problems such as noise, dust and the proximity of sensitive equipment. Blast cleaning of structures previously coated with lead-based paints presents legal and health risks associated with the containment and the disposal of the lead-containing paint flakes within the dust and blast residue. Thus, corrosion protection is extremely costly due to the costliness of abrasive blast cleaning to high standards. Corrosion protection over the long term is often impossible in situations where blast cleaning is not possible.